Metal oxide semiconductor materials having high band gap are used as, for example, photoelectric conversion materials, photocatalytic materials, optical sensors and storage battery materials (batteries).
Of these, the photoelectric conversion materials are those from which optical energy can be continuously taken out as electrical energy and are those to convert optical energy into electrical energy using electrochemical reaction between electrodes. When such photoelectric conversion materials are irradiated with light, electrons are generated on one electrode side and move to the other electrode, and then the electrons migrate in the electrolyte as ions and return to the first electrode. Since this energy conversion is continuously made, it can be utilized for, for example, solar cells.
A common solar cell comprises a substrate such as a glass plate having a transparent conductive film as an electrode and a semiconductor film for a photoelectric conversion material thereon, and includes another substrate (e.g., glass plate) having a transparent conductive film as the other electrode, and an electrolyte enclosed between the electrodes.
When a photosensitizer adsorbed on the semiconductor is irradiated with, for example, sunlight, the photosensitizer absorbs light of visible region and is excited. The electrons generated by the excitation move to the semiconductor and to the transparent conductive glass electrode. Then, the electrons move to the other electrode through a conductor connecting the two electrodes. The electrons having moved to the other electrode reduce the oxidation-reduction system in the electrolyte. On the other hand, the photosensitizer having caused the electrons to move to the semiconductor is in a state of an oxidant, and this oxidant is reduced by the oxidation-reduction system in the electrolyte to return to its original state. Thus, the electrons continuously flow, and the photoelectric conversion material functions as a solar cell.
As the photoelectric conversion materials, those wherein a spectrosensitizing dye having absorption in the visible region is adsorbed on the semiconductor surface have been employed. For example, Japanese Patent Laid-Open Publication No. 220380/1989 describes a solar cell having a spectrosensitizing dye layer made of a transition metal complex such as a ruthenium complex on a surface of a metal oxide semiconductor. In National Publication of International Patent No. 504023/1993 (corresponding to WO91/16719), a solar cell having a spectrosensitizing dye layer made of a transition metal complex such as a ruthenium complex on a surface of a titanium oxide semiconductor layer doped with metal ion is described.
For such solar cells, it is important, from the viewpoint of improvement in the photoelectric conversion efficiency, that the electrons rapidly move to the titanium oxide semiconductor layer from the layer of the spectrosensitizing dye such as a ruthenium complex, which has been excited by the light absorption. If the electrons do not move rapidly, recombination of the electrons with the ruthenium complex or reverse current (also referred to as “reverse current” or “back current”) of the electrons takes place, resulting in a problem of lowering of the photoelectric conversion efficiency.
On this account, increase of the amount of the spectrosensitizing dye adsorbed on the surface of the titanium oxide semiconductor film or improvement of mobility of the electrons in the titanium oxide semiconductor film has been studied.
For example, it has been proposed that in the formation of the titanium oxide semiconductor film, steps of applying a titania sol onto an electrode substrate, drying the sol and calcining the dried film are repeated to form a porous thick film, that is, a porous semiconductor film is formed to increase the amount of the Ru complex supported on the surface of the semiconductor film. It has been also proposed that the titania fine particles are calcined at a temperature of not lower than 400° C. to improve conductivity. Moreover, it has been proposed in National Publication of International Patent No. 511113/1994 (corresponding to WO93/18532) that, in order to increase the effective surface area, immersion of the film in an aqueous solution of titanium chloride or deposition of titanium chloride on the film is carried out to form a porous titanium oxide layer.
In the above processes, however, calcining of the titanium oxide semiconductor film is carried out to improve the electron mobility. Therefore, the particles are sintered to cause a lowering of porosity, so that there resides a problem that the adsorbed amount of the spectrosensitizing dye is decreased. Thus, the photoelectric conversion efficiency is not always satisfactory, and further improvement has been desired.
The present inventors have disclosed a process for producing titanium oxide semiconductor films for photovoltaic cells and a photovoltaic cell, which have been improved in the above points (Japanese Patent laid-Open Publication No. 339867/1999).
Further, the present inventors have disclosed a photovoltaic cell which uses, in order to inhibit the back current, a metal oxide semiconductor film composed of metal oxide particles having core-shell structure wherein the volume resistance value of the core is lower than that of the shell (Japanese Patent laid-Open Publication No. 155791/2001).
The photoelectric conversion efficiency of the above photovoltaic cells has been improved to a certain extent, but there is a problem that the electromotive force is insufficient or the electromotive force and the photoelectric conversion efficiency are lowered after the cells are used for a long period of time.
In the conventional photovoltaic cells, the adsorbed amount of the spectrosensitizing dye (photosensitizer) is not always large, so that back current is liable to take place, and the photoelectric conversion efficiency is not high. Moreover, the photovoltaic cells tend to absorb ultraviolet rays, and therefore, if they are used for a long period of time, the photosensitizer is gradually decomposed. As a result, the electromotive force and the photoelectric conversion efficiency of the photovoltaic cells are gradually decreased, and the durability of the photovoltaic cells becomes insufficient.
The present invention is intended to solve such problems as described above by providing a photovoltaic cell capable of inhibiting back current and decomposition of a spectrosensitizing dye caused by the ultraviolet rays, possessing high photoelectric conversion efficiency and which is capable of generating high electromotive force.